1. Field of the Invention
The present invention relates to a wireless transmitter, a wireless receiver, and a wireless communication system that exist in an environment where a plurality of user groups or a plurality of service providers (these will be referred to by the general term ‘a plurality of groups’ hereinbelow) perform wireless communications simultaneously by using a common frequency band, and the present invention relates to a wireless signal control method that is executed by the wireless transmitter, the wireless receiver, and the wireless communication system.
2. Related Background Art
Communication by a plurality of groups using a common frequency band has been examined as a possible method for equitably utilizing wireless resources. An essential condition for a plurality of groups commonly using the same frequency is that they perform communications equitably without obstructing the communications of other groups.
A wireless LAN system may be given as an example of conventional frequency sharing technology. In the case of a wireless LAN system, a plurality of wireless stations autonomously implement the co-existence of their wireless signals over the same frequency in an environment where a plurality of wireless stations coexist by performing carrier sensing to judge whether a predetermined frequency is being used by another station in the vicinity and transmitting a signal when the predetermined frequency is not being used in the vicinity (CSMA/CA). In CSMA/CA, once the busy state where a frequency is being used ends, the wireless station transmits a signal after waiting for a time obtained by adding a random amount of time to a fixed time interval. Because each user starts communicating after waiting for a time obtained by adding a random amount of time in this manner, the probability of each user being able to perform communication is equitable, and thus the fairness of communication between users can be maintained (see “‘IEEE Std 802.11a-1999” The Institute of Electrical and Electronics Engineers, 1999’).
As a wireless system where a plurality of service providers commonly use a plurality of wireless channels that exist in the same frequency band, there exists PHS. With PHS, a plurality of service providers commonly use a plurality of prepared carriers for communication. PHS is used with a base station performing call control for each service provider by means of a control carrier that is individually prepared for each service provider and selecting the carrier for communication to be used from the unused carriers for communication. Regarding the selected common carrier for communication, fairness between users is maintained by means of time division multiple access (TDMA), in which each user is permitted to use the frequency for transmission of a data signal only during the assigned time slot. The time slot is obtained by dividing a certain time period in advance (See “ARIB RCR STD-28” Association of Radio Industries and Businesses, 1993).
However, for the wireless LAN system, it is possible to obtain fairness between users by means of CSMA/CA, but in the case where there is a user that breaches etiquette and continues to use the wireless resources, it becomes impossible for other users to utilize that frequency band. Also, in an environment where a plurality of groups coexists, there is a problem that a group with few users has a diminished probability of being able to establish communication compared to a group with a large number of users, and, hence, fairness between groups cannot be guaranteed. For this reason, in a wireless LAN system, it becomes important to assign IDs that serve to discriminate between users occupying the frequency or the groups to which the users belong, to the wireless signals in order to control etiquette breaches and ensure fairness between service providers.
Furthermore, in the case of the PHS, in addition to the above-mentioned problems, because control of communication is performed by preparing an exclusive control carrier for a plurality of groups, it becomes difficult to assign each group a control carrier as the number of groups increases, because the frequency resources are finite. For this reason, it becomes important to achieve a technology that assigns the ID of the groups to which each wireless signal belongs and classifies the group that the received wireless signal belongs to without requesting excessive frequency resources.
When it is possible to discriminate between groups, for example, in the case of a wireless LAN system using CSMA/CA, it also becomes possible to control the fairness between groups by controlling the random times on the basis of statistic quantities (for example, the proportion of how many times each group delivers a signal in a predetermined time interval before a signal is sent, or the like) obtained from the results of group discrimination (see “IEEE Std 802.11e-2005” The Institute of Electrical and Electronics Engineers, 2005).
As per a communication system such as PHS that uses TDMA/TDD, by controlling the transmission slot and frequency allocation opportunity based on the statistic quantities obtained from the group discrimination result, the fairness between groups can be controlled.
In a wireless LAN system, in order to assign the ID of the group, to which a wireless signal belongs, to the wireless signal, for example, as shown in FIG. 1, a wireless transmitter 11 transmits a wireless signal 13 after inserting an ID (wireless signal ID: MAC address, for example), which serves to identify the group to which the wireless signal 13 belongs, into the wireless signal 13. At the wireless receiver 12, the wireless signal 13 is divided into a wireless signal ID and a data signal by a signal divider 14, and the wireless signal ID is demodulated. After that, at an ID discriminator 15, the group, to which the received signal 13 belongs, is discriminated based on the demodulated wireless signal ID.